Frequency synthesizers have typically been provided with reference frequency signals from crystal controlled frequency oscillators having temperature compensation circuits for use in frequency stabilized radio communications. These compensation circuits may be composed of analog or digital devices and are used to provide a relatively flat frequency output over temperature. Typically, a capacitive element is provided to allow absolute adjustment of the final frequency of the oscillator. This capacitive element is generally either in the form of a trim capacitor or an analog varactor controlled by an applied DC voltage. By adjusting this capacitance, the user is able to adjust (warp) the oscillator onto a desired frequency.
A typical radio application includes at least one RF local oscillator. The more sophisticated local oscillators consist of a frequency synthesizer which can include a temperature compensated crystal oscillator (TCXO) coupled to a phase locked loop (PLL) which includes a voltage controlled oscillator (VCO) and a loop filter. Furthermore, in one of today's dual band radios, one can find a second VCO and possibly a second PLL. Moreover, additional PLLs can be used for separate RF and IF functions in the radio.
In typical frequency synthesizer applications multiple discrete elements are soldered to a circuit board to make up a functioning frequency synthesizer. These numerous elements presently take up a considerable amount of circuit board space in the radio. It would be advantageous to provide a frequency synthesizer that takes up much less circuit board space. In particular, it would be very beneficial to provide a dual band frequency synthesizer without the circuit board clutter of multiple discrete components and without the multiple affixing processes needed for these components.
There have been prior art attempts to integrate frequency synthesizers on a silicon chip, however these prior art devices fail to integrate the entire function of the frequency synthesizer in one device as some of the needed components to make the synthesizer function can not be integrated. A first prior art circuit includes a fractional-N phase lock loop and modulator/demodulator on a single integrated circuit. However, this device does not include an on-board TCXO or VCO. A second prior art circuit includes a frequency synthesizer and a gain block portion of a VCO. However, this device does not include an on-board crystal or the tank circuit for the VCO.
Therefore, there is a need for a stand alone frequency synthesizer which: is modularized to provide a single component for the radio designer, is temperature compensated, achieves equivalent or better performance than prior art frequency synthesizers; is easily controlled by a single data bus, and has all necessary on-board components. In addition, it is desirable to provide a low cost, small sized, low current drain frequency synthesizer that allows control of the temperature compensation of the crystal oscillator while providing low phase noise.